Electrical power supplied by the utility grid is not sufficiently reliable for an increasing number of applications. Power interruptions and voltage sags often cause substantial problems in electrical and electromechanical devices that limit the effectiveness and reliability of these devices. R. T. Morash and R. J. Barber of Precise Power Corporation published prior to 1997 a study entitled “How to Improve Your Customers' Power Quality.” In this study, they delineated three bell-shaped, that is, normally distributed, curves that describe the quality of power that can be expected from the grid. They found that 90% of power interruptions have durations of between 0.1 seconds and 10 seconds, 5% of power interruptions have mean durations of approximately 10 seconds and 5% of power interruptions have mean durations of approximately 3 hours. In a separate study, H. Reiss in 1992 in an ABB Technical Report found at one U.S. facility that of the 443 power disturbances that occurred in one year: 264 were sags (voltage amplitude falls below nominal for one or more cycles) and 15 were interruptions (complete loss of voltage—can be momentary, a few power cycles, or prolonged.) In total, the grid is 99.9% reliable, indicating that the loss of power will occur at an average location for 8 hours per year.
If a power conditioning device, in particular, a flywheel uninterruptible power device is used, inertial energy will be periodically extracted from the flywheel and converted to electrical energy in order temporarily to supplement or make up for power normally derived from the grid. This extracted power will cause the rotational velocity and thereby the stress level in the flywheel to cycle from a maximum value to a lower value. Once the power disturbance has passed, some power from the grid is used to accelerated the flywheel back to its maximum operational rotational velocity, thus completing the discharge/charge cycle. Since the stress level in the flywheel will vary according to the duration of the power disturbance and since the aforementioned studies indicate that the duration of power interruptions vary over a wide range, the longevity of a flywheel uninterruptible power supply cannot be assumed or simply determined. The longevity of the flywheel, in fact, will be a complex function of the history of the power disturbances.
Flywheel energy storage systems are also utilized for other applications besides uninterruptible power supplies. Such applications include bulk energy storage, energy storage from alternative energy generation, load leveling, hybrid electric vehicle acceleration and regeneration energy storage, and for pulse power applications. No matter the use, ensuring the safety and reliability of flywheel energy storage systems are of critical importance.